The present invention refers to transgenic plants and plant cells, which have been transformed with the soybean calmodulin isoform genes (SCaM4 and SCaM5) to exhibit greatly enhanced resistance to a wide spectrum of plant pathogens. The present invention also provides the expression vector containing said genes and to host cell into which said genes in the expression vector have been introduced to plant pathogens-resistant plants. Transgenic plants expressing a heterologous SCaM4 or SCaM5 show increased resistance to fungi, bacteria and viruses which normally infect the plants.
Plants are constantly being challenged by aspiring pathogens. There are great economic losses caused by pathogenic attacks against higher plants in which the natural defenses of plants are inadequate or fail to respond and defend the plants against damage by pathogens. Therefore, control of various plant pathogens is very important in agriculture. Extensive efforts have been focused on controlling pathogenic diseases in crops. However, little success has been achieved by breeding programs to select for crops that are more resistant to pathogens. Furthermore, successful pathogen invasions and diseases ensue if the preformed plant defenses are inappropriate, the plant does not detect the pathogens, or the activated defense responses are ineffective.
It is well known that the resistance of plants to invading pathogens is accompanied by the deployment of a complex array of defense responses (Jackson et al. 1996. Plant Cell 8:1651-1668). When the pathogen carries a specific avirulence (avr) gene and the plant host contains a cognate resistance, the formation of local lesions occurs at the site of infection that results in inhibition of pathogen growth (termed the hypersensitive response; HR) (The Hypersensitive Reaction in Plants to Pathogens: A Resistance Phenomenon. Goodman, R. N. and Novacky, A. J., APS Press, St. Paul. 1994). Therefore, a plant expressing a particular resistance (R) gene is specifically resistant to pathogens expressing the corresponding avr gene. In addition to the hypersensitive response, a secondary defense response can be triggered that renders uninfected parts of the plant resistant to a variety of virulent pathogens (termed systemic acquired resistance; SAR). The interactions between plants and pathogens lead to a series of defense signal transduction events, including oxidative burst, transient Ca.sup.+2 increase, salicylic acid accumulation, the synthesis of high levels of pathogenesis-related proteins, phytoalexin biosynthesis and defense gene activations. Accumulating evidence implicates the involvement of a Ca.sup.2+ signal in certain plant defense responses. A Ca.sup.2+ ion influx is one of the earliest events in challenged cells (Dixon et al. 1994. Annu. Rev. Phytopathol. 32:479-501) and has been shown to be essential for the activation of defense responses such as phytoalexin biosynthesis, induction of defense-related genes, and hypersensitive cell death (Levine et al. 1996. Curr. Biol. 6:427-437). However, direct evidences for the involvement of CaM in plant disease resistance responses have been unavailable.
Various approaches have been utilized for attempting to control deleterious fungi, bacteria, viruses and even nematodes. One approach is the application of certain naturally occurring bacteria which inhibit or interfere with fungi or nematodes. Another approach is breeding for resistance, which is primarily focused on the manipulation of minor resistance genes which make small quantitative contributions to the overall resistance of the plant. However, there is often an inability of the plants to recognize the pathogen to cause the. defenses of the plants to be induced. Furthermore, the protection provided by these approaches is much narrower than that rendered by full-fledged systemic acquired resistance, and the degree of resistance is much less significant.
Transgenic plants of this invention contain similar levels of specific divergent SCaM4 or SCaM5 protein (0.3-0.5 .mu.g/mg total soluble leaf protein) to that of the highly conserved CaM isoforms (SCaM1, SCaM2, and SCaM3) in wild type plants. Upon challenge with pathogens, it has been found that the transgenic plants provided by this invention show long-lasting, broad-spectrum resistance against a variety of pathogens (fungal, viral and bacterial pathogens), similar to systemic acquired resistance. To our knowledge, this is the first in vivo evidence not only for the functional differences among CaM isoforms but also for a central role of SCaM4 and SCaM5 for a broad spectrum of pathogen (including virus, bacteria and fungi) resistance in plants.